Radio communication system, radio base station, and communication control method that can reduce an inter-base station interference between downlink control channels

ABSTRACT

A picocell base station (PeNB) uses a downlink control channel, the frequency band of which overlaps with the frequency band of the downlink control channel used by a macrocell base station (MeNB), to transmit downlink control information for controlling radio communication with a radio terminal (PUE) connected to the picocell base station (PeNB). The picocell base station (PeNB) comprises an X2 interface communication unit ( 140 ) which uses inter-base station communication to transmit to the macrocell base station (MeNB) usage control information for controlling the control channel usage that is the usage of a radio resource used as the downlink control channel by the macrocell base station (MeNB).

TECHNICAL FIELD

The present invention relates to a radio communication system, a radiobase station, and a communication control method which transmit downlinkcontrol information using a downlink control channel.

BACKGROUND ART

The next generation systems achieving higher-speed and larger-capacitycommunications than the currently-operated 3rd generation and 3.5thgeneration cellular radio communication systems include LTE (Long TermEvolution) and LTE Advanced being an advanced version of LTE, which arestandardized by the standardization organization 3GPP.

In the LTE system (including the LTE Advanced), a radio base stationtransmits downlink control information (DCI: Downlink ControlInformation) for controlling radio communications with a radio terminalby using a downlink control channel (PDCCH: Physical Downlink ControlCHannel). There is an overlap in frequency band of the downlink controlchannel between radio base stations.

In the LTE Advanced, discussion is in progress on provision of aheterogeneous network in which there both exist a high-power basestation (a so-called macrocell base station) and a low-power basestation (a so-called picocell base station or a femtocell base station).The heterogeneous network can distribute a load of the high-power basestation to the low-power base station.

In general, a radio terminal connects with a radio base station thatprovides a radio signal with the highest received power among multipleradio base stations. Thus, in the heterogeneous network, a radioterminal is less likely to connect with the low-power base station.Against these circumstances, there has been proposed a technique ofexpanding the self-coverage of a low-power base station by controlling aradio terminal so that the radio terminal will connect with thelow-power base station even when the received power from the low-powerbase station is not the highest (see, for example, NON-PATENT DOCUMENT1).

PRIOR ART DOCUMENT Non-Patent Document

-   NON-PATENT DOCUMENT 1: 3GPP R1-10506, “Importance of Serving Cell    Selection in Heterogeneous Networks,” February, 2010.

SUMMARY OF THE INVENTION

Since there is an overlap in frequency band of the downlink controlchannel between radio base stations, the downlink control channel usedby one of two neighboring radio base stations receives interference fromthe downlink control channel used by the other radio base station, andtherefore the downlink control information on the downlink controlchannel of the one radio base station cannot be normally received. Ifthe downlink control information cannot be normally received from aradio base station, a radio terminal has difficulty in performing radiocommunications with the radio base station.

In particular, in the technique of expanding the self-coverage of alow-power base station in a heterogeneous network, the downlink controlchannel used by the low-power base station is highly likely to receiveinterference from the downlink control channel used by a high-power basestation. Thus, the above-described problem is more serious.

Accordingly, an objective of the present invention is to provide a radiocommunication system, a radio base station, and a communication controlmethod which can reduce an inter-base station interference betweendownlink control channels.

The present invention has the following features to solve the problemsdescribed above. A feature of a radio communication system according tothe present invention is summarized as follows. A radio communicationsystem, comprises: a first radio base station configured to controlradio communications with a radio terminal connecting with the firstradio base station by using a downlink control channel; and a secondradio base station which is a neighboring radio base station of thefirst radio base station and is configured to control radiocommunications with a radio terminal connecting with the second radiobase station by using a downlink control channel having a frequency bandoverlapping with a frequency band of the downlink control channel usedby the first radio base station, wherein the first radio base stationincludes a transmitter configured to send the second radio base stationcontrol information for controlling a usage state of the downlinkcontrol channel of the second radio base station, and the second radiobase station includes: a receiver configured to receive the controlinformation from the first radio base station; and a controllerconfigured to control the usage state of the downlink control channel ofthe second radio base station in accordance with the control informationreceived by the receiver.

With such feature, the first radio base station can reduce interferenceof own downlink control channel from the downlink control of the secondradio base station by controlling the usage state of the downlinkcontrol of the second radio base station. Therefore, a user terminalconnecting to the first radio base station can receive downlink controlinformation normally even when the first radio base station expands theown cell coverage.

Another feature of the radio communication system according to thepresent invention is summarized as follows. In the radio communicationsystem according to the aforementioned feature, the usage state is acontrol channel usage indicating an amount of radio resource used as thedownlink control channel.

Another feature of the radio communication system according to thepresent invention is summarized as follows. In the radio communicationsystem according to the aforementioned feature, the control informationcontains information indicating a limitation target subframe which is asubframe in which the control channel usage of the first radio basestation or the second radio base station is to be limited to an upperlimit value or below.

Another feature of the radio communication system according to thepresent invention is summarized as follows. In the radio communicationsystem according to the aforementioned feature, the control informationcontains information indicating the upper limit value.

Another feature of the radio communication system according to thepresent invention is summarized as follows. In the radio communicationsystem according to the aforementioned feature, the control informationcontains information on whether to make linked-control of a data channelusage indicating an amount of radio resource used as a downlink datachannel according to the control channel usage in the limitation targetsubframe.

Another feature of the radio communication system according to thepresent invention is summarized as follows. In the radio communicationsystem according to the aforementioned feature, the control informationcontains information indicating that the first radio base station limitsdownlink transmission power of all frequency bands in a data region inthe limitation target subframe to a threshold or below, and informationindicating the threshold.

Another feature of the radio communication system according to thepresent invention is summarized as follows. In the radio communicationsystem according to the aforementioned feature, the control informationcontains symbol number information indicating the number of symbolscorresponding to a time range, which are used by the first radio basestation as the downlink control channel within a subframe, and usagerate information indicating a usage rate of radio resource, which is anupper limit for the second radio base station within the time range.

Another feature of the radio communication system according to thepresent invention is summarized as follows. In the radio communicationsystem according to the aforementioned feature, the control informationcontains symbol number information indicating the number of symbolscorresponding to a time range, which are used by the first radio basestation as the downlink control channel within a subframe, and usagerate information indicating a usage rate of radio resource, which is anupper limit for the first radio base station within the time range.

Another feature of the radio communication system according to thepresent invention is summarized as follows. In the radio communicationsystem according to the aforementioned feature, the controller of thesecond radio base station controls the radio resource usage rate of thesecond radio base station within the time range according to the symbolnumber information and the usage rate information which are contained inthe control information received by the receiver.

Another feature of the radio communication system according to thepresent invention is summarized as follows. In the radio communicationsystem according to the aforementioned feature, the usage state istransmission power of the downlink control channel.

A feature of a radio base station according to the present invention issummarized as follows. A radio base station configured to control radiocommunications with a radio terminal connecting with the radio basestation by using a downlink control channel having a frequency bandoverlapping with a frequency band of a downlink control channel used bya neighboring base station, comprises: a transmitter configured to sendthe neighboring base station control information for controlling a usagestate of the downlink control channel of the neighboring base station.

A feature of a radio base station according to the present invention issummarized as follows. A radio base station configured to control radiocommunications with a radio terminal connecting with the radio basestation by using a downlink control channel having a frequency bandoverlapping with a frequency band of a downlink control channel used bya neighboring base station, comprises: a receiver configured to receivethe control information for controlling a usage state of the downlinkcontrol channel of the radio base station, and a controller configuredto control the usage state of the downlink control channel of the radiobase station according to the control information received by thereceiver.

A feature of a communication control method according to the presentinvention is summarized as follows. A communication control method usedin a radio communication system comprises: a first radio base stationconfigured to control radio communications with a radio terminalconnecting with the first radio base station by using a downlink controlchannel; and a second radio base station which is a neighboring radiobase station of the first radio base station and is configured tocontrol radio communications with a radio terminal connecting with thesecond radio base station by using a downlink control channel having afrequency band overlapping with a frequency band of the downlink controlchannel used by the first radio base station, the communication controlmethod comprising the steps of: sending control information forcontrolling a usage state of the downlink control channel of the secondradio base station from the first radio base station to the second radiobase station; receiving by the second radio base station the controlinformation from the first radio base station; and controlling by thesecond radio base station a usage state of the downlink control channelof the second radio base station according to the control informationreceived at the receiving step.

The features of the present invention can provide a radio communicationsystem, a radio base station, and a communication control method, whichcan reduce an inter-base station interference between downlink controlchannels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing for illustrating a summary of the LTE systemaccording to first to seventh embodiments.

FIG. 2(a) is a frame configuration diagram showing a downlink radioframe configuration when the FDD scheme is used, and FIG. 2(b) is aframe configuration diagram showing a downlink subframe configuration.

FIG. 3 is a schematic configuration diagram of a radio communicationsystem according to the first embodiment.

FIG. 4 is a block diagram showing the configuration of a picocell basestation according to the first embodiment.

FIG. 5 is a block diagram showing the configuration of a macrocell basestation according to the first embodiment.

FIG. 6 is a drawing for illustrating a configuration example of usagecontrol information according to the first embodiment.

FIG. 7 is an operational sequence diagram showing an operational exampleof a radio communication system according to the first embodiment.

FIG. 8 is a schematic configuration diagram of a radio communicationsystem according to the second embodiment.

FIG. 9 is an operational sequence diagram showing an operational exampleof the radio communication system according to the second embodiment.

FIG. 10 is a block diagram showing the configuration of a picocell basestation according to the third embodiment.

FIG. 11 is a block diagram showing the configuration of a macrocell basestation according to the third embodiment.

FIG. 12 is a drawing showing a configuration example of a PDCCHnotification message according to the third embodiment.

FIG. 13 is an operational sequence diagram showing an operationalexample of the radio communication system according to the thirdembodiment.

FIG. 14 is a block diagram showing the configuration of a picocell basestation according to the fourth embodiment.

FIG. 15 is an operational sequence diagram showing an operationalexample of the radio communication system according to the fourthembodiment.

FIG. 16 is a block diagram showing the configuration of a picocell basestation according to the fifth embodiment.

FIG. 17 is an operational sequence diagram showing an operationalexample of the radio communication system according to the fifthembodiment.

FIG. 18 is a drawing for illustrating an interference state of apicocell base station with respect to PDCCH according to the sixthembodiment.

FIG. 19 is a drawing for illustrating an example of a messageconfiguration according to the sixth embodiment.

FIG. 20 is a block diagram showing the configuration of a macrocell basestation according to the seventh embodiment.

FIG. 21 is a block diagram showing the configuration of a picocell basestation according to the seventh embodiment.

FIG. 22 is a drawing for illustrating an operational example of theradio communication system according to the seventh embodiment.

DESCRIPTION OF THE EMBODIMENTS

Referring to the drawings, first to seventh embodiments and otherembodiments of the present invention are described. In the drawings inthe embodiments, same or similar reference numerals are given to denotesame or similar portions.

[Summary of LTE System]

Before radio base stations according to first to seventh embodiments aredescribed, the summary of the LTE system to which the radio basestations are applied is described with regard to the contents relatingto the present invention.

FIG. 1 is a drawing for illustrating the summary of the LTE system. Asshown in FIG. 1, multiple radio base stations eNB form E-UTRAN(Evolved-UMTS Terrestrial Radio Access Network). Each of the radio basestations eNB forms a cell which is an area where to provide acommunication service to a radio terminal UE.

The radio terminal UE is a radio communication apparatus carried by auser, and is also referred to as user equipment. The radio terminal UEconnects with one of the radio base stations eNB whose received power ofa reference signal (RSRP: Reference Signal Received Power) is thehighest. However, not only the RSRP, another receiving quality indexsuch as an SNR (Signal to Noise ratio) may be used.

The radio base stations eNB are capable of communicating with oneanother via an X2 interface which is a logical communication path toprovide communications between the base stations. Each of the radio basestations eNB is capable of communicating with EPC (Evolved Packet Core),specifically, an MME (Mobility Management Entity)/(S-GW (ServingGateway)) via an S1 interface.

In the radio communications between the radio base station eNB and theradio terminal UE, the OFDMA (Orthogonal Frequency Division MultipleAccess) scheme and SC-FDMA (Single-Carrier Frequency Division MultipleAccess) scheme are respectively applied as a downlink multiplex mode andan uplink multiplex mode. In addition, the FDD (Frequency DivisionDuplex) scheme or TDD (Time Division Duplex) scheme is applied as acomplex communication mode.

FIG. 2(a) is a frame configuration diagram showing a downlink radioframe configuration when the FDD scheme is used.

As shown in FIG. 2(a), the downlink radio frame is formed of 10 downlinksubframes and each downlink subframe is formed of 2 downlink slots. Eachdownlink subframe has a length of 1 ms and each downlink slot has alength of 0.5 ms. In addition, each downlink slot includes 7 OFDMsymbols (transmission symbols) in the time axis direction (a timedomain) and multiple resource blocks (RBs) in the frequency axisdirection (a frequency domain). Each RB includes 12 sub-carriers.

FIG. 2(b) is a frame configuration diagram showing the configuration ofthe downlink subframe.

As shown in FIG. 2(b), the downlink subframe includes two continuousdownlink slots. A section from the head of the first downlink slot inthe downlink subframe to a maximum of 3 OFMD symbols is a control regionforming radio resource which is used as PDCCH for transmitting controlinformation DCI. The DCI corresponds to uplink and downlink schedulinginformation. Note that described herein is that a maximum of 3 OFDMsymbols are used, but a maximum of 4 symbols can be used if the systembandwidth is narrow.

A remaining OFDM symbol section of the downlink subframe is a dataregion forming radio resource which is used as PDSCH (Physical DownlinkShared Channel) for transmitting data information. Note that the controlregion may include PCFICH (Physical Control Format Indicator Channel)and PHICH (Physical Hybrid Automatic Repeat Request Indicator Channel)in addition to the PDCCH.

The radio terminal UE can specify data information which is transmittedvia the PDSCH by decoding the DCI transmitted by the PDCCH. The radioterminal UE can specify the number of OFDM symbols in the time axisdirection in the control region within the downlink subframe with thePCFICH.

The transmission in the PDCCH is performed for each resource unitincluding 36 symbols referred to as CCE (Control Channel Elements). Inother words, the radio base station eNB can control a coding rate bycontrolling the number of allocated CCEs for each radio terminal UE. Theradio terminal UE which can decode even at a high coding rate caneffectively utilize the radio resource by reducing the number of theallocated CCEs. The radio terminal UE performs blind decoding,specifically, decoding on all the numbers of CCEs and detects the numberof CCEs which can be correctly decoded as the number of allocated CCEs.

First Embodiment

Hereinafter, the first embodiment of the invention is described. Thefirst embodiment is an embodiment when the present invention is appliedto a heterogeneous network.

(1) Configuration of Radio Communication System

A schematic configuration of a radio communication system according tothe first embodiment is described. FIG. 3 is a schematic configurationdiagram of a radio communication system 1A according to the firstembodiment.

As shown in FIG. 3, the radio communication system 1A has a macrocellbase station MeNB (a high-power base station or high-output basestation), a radio terminal MUE connected with the macrocell base stationMeNB, a picocell base station PeNB which is installed within a macrocellformed by the macrocell base station MeNB and neighbors on the macrocellbase station MeNB, and a radio terminal PUE connected with the picocellbase station PeNB within a picocell PC formed by the picocell basestation PeNB (a low-power base station or a low-output base station).

The picocell base station PeNB (also referred to as a hot zone node) isa low-power base station whose transmission output (maximum transmissionpower) is smaller than that of the macrocell base station MeNB.Accordingly, in the heterogeneous network, the picocell base stationPeNB (a range capable of accommodating the radio terminal UE) may have asmall self-coverage with application of a selection standard in whichthe radio terminal UE selectively connects with the radio base stationeNB having the highest RSRP. In particular, under the situation wherethe picocell base station PeNB is close to the macrocell base stationMeNB, the self-coverage of the picocell base station PeNB becomesextremely small. Thus, the picocell base station PeNB cannot beeffectively utilized.

The following two methods can be mainly used as methods in which theself-coverage of the picocell base station PeNB can be expanded withoutincreasing the transmission output of the picocell base station PeNB.

First, there is a method of selecting a radio base station having thesmallest propagation loss (path loss) with a radio terminal UE as aconnection destination for the radio terminal UE without selecting theradio base station eNB transmitting a radio signal having the largestRSRP as a connection destination for the radio terminal UE. This canexpand the self-coverage of the picocell base station PeNB because, forexample, the radio base station eNB nearest to the radio terminal UE isselected as the connection destination.

Second, there is a method in which when a radio terminal UE can receivea radio signal from each of the macrocell base station MeNB and thepicocell base station PeNB and compares the RSRP of the picocell basestation PeNB and the RSRP of the macrocell base station MeNB with eachother, the radio terminal UE adds an offset value to the RSRP of thepicocell base station PeNB. The offset value is added to the RSRP of thepicocell base station PeNB, which increases a possibility that the RSRPafter the offset will exceed the RSRP of the macrocell base stationMeNB. Accordingly, the self-coverage of the picocell base station PeNBcan be expanded because the picocell base station PeNB is preferentiallyselected as a connection destination. Instead of the addition of theoffset value to the RSRP of the picocell base station PeNB, asubtraction of an offset value from the RSRP of the macrocell basestation MeNB can also produce a similar effect.

In the first embodiment, it is assumed that the self-coverage of thepicocell base station PeNB is in an expanded state by any one of theabove two methods or another method. Note that a subject of selectingthe radio terminal UE is the radio terminal UE if the radio terminal UEis in stand-by (an idle state) and is the radio base station eNB in aconnection destination if the radio terminal UE is in communication (anactive state). In the active state, a measured RSRP value isperiodically reported from the radio terminal UE to the radio basestation eNB in the connection destination. Thus, the radio base stationeNB in the connection destination can select the next connectiondestination of the radio terminal UE.

The macrocell base station MeNB transmits DCI for controlling radiocommunications with the radio terminal MUE by using PDCCH. The picocellbase station PeNB transmits DCI for controlling radio communicationswith the radio terminal PUE by using PDCCH. Frequency bands of thesePDCCHs overlap with each other. In addition, the respective PDCCHs ofthe macrocell base station MeNB and the picocell base station PeNB alsooverlap with each other in the time axis. As a result, the respectivePDCCHs of the macrocell base station MeNB and the picocell base stationPeNB interfere with each other.

In the state where the self-coverage of the picocell base station PeNBis expanded, the radio terminal PUE connecting with the picocell basestation PeNB is sometimes in a state where received power (RSRP) fromthe macrocell base station MeNB is higher than received power (RSRP)from the picocell base station PeNB. In this case, the PDCCH which isused by the picocell base station PeNB receives a large interferencefrom the PDCCH which is used by the macrocell base station MeNB. Thus,the radio terminal PUE cannot receive (decode) the DCI.

For this reason, in the first embodiment, the picocell base station PeNBtransmits usage control information for controlling a PDCCH usage whichis an amount of radio resources used by the macrocell base station MeNBas PDCCH to the macrocell base station MeNB via the X2 interface. Here,the PDCCH usage means, for example, the number of OFDM symbols in thetime axis direction in the above-described control region or a CCEamount in the control region. In addition, the PDCCH usage is a conceptincluding a PDCCH usage rate. The PDCCH usage in the macrocell basestation MeNB is reduced, so that the PDCCH used by the picocell basestation PeNB is reduced in interference received by the PDCCH used bythe macrocell base station MeNB.

(2) Configuration of Picocell Base Station

Hereinafter, the configuration of the picocell base station PeNB isdescribed. FIG. 4 is a block diagram showing the configuration of thepicocell base station PeNB.

As shown in FIG. 4, the picocell base station PeNB has an antenna unit101, a radio communication unit 110, a controller 120, a storage unit130, and an X2 interface communication unit 140. In the firstembodiment, the X2 interface communication unit 140 corresponds to atransmitter to transmit control information.

The radio communication unit 110 is configured using, for example, aradio frequency (RF) circuit and a base band (BB) circuit, andtransmits/receives a radio signal to/from the radio terminal PUE via theantenna unit 101. Also, the radio communication unit 110 modulates thetransmission signal and demodulates the received signal.

The controller 120 is configured using, for example, a CPU and controlsvarious kinds of functions included in the picocell base station PeNB.The storage unit 130 is configured using, for example, a memory, andstores various pieces of information to be used for controlling thepicocell base station PeNB and the like. The X2 interface communicationunit 140 performs inter-base station communications with other radiobase stations using the X2 interface.

The controller 120 has a target subframe determination unit 121, a PDCCHusage determination unit 122, and a usage control information generator123.

The target subframe determination unit 121 determines a target subframeper one radio frame to be a target in which a PDCCH usage of themacrocell base station MeNB is controlled. In other words, the usagecontrol information is configured so that the PDCCH usage can bedesignated per a downlink subframe. For example, the target subframedetermination unit 121 may determine a target subframe according to aratio of UEs requiring suppression of PDCCH interference with respect tomultiple radio terminals PUEs connected with the picocell base stationPeNB. For example, the target subframe determination unit 121 increasesthe number of target subframes as the ratio of UE requiring thesuppression of the PDCCH interference becomes higher. Note that, thetarget subframe determination unit 121 can specify the radio terminalPUE requiring the suppression of the PDCCH interference with receptionquality information (such as a measurement report) which is reportedfrom each radio terminal PUE. Furthermore, when the PDCCH of themacrocell base station MeNB is controlled by another picocell basestation, it is preferable that the target subframe determination unit121 coordinate the control of its own picocell base station with thecontrol of the other picocell base station. In other words, the targetsubframe determination unit 121 determines the same subframe as thetarget subframe determined by the other picocell base station. To thispurpose, the macrocell base station MeNB or the other picocell basestation may be configured to notify, to the picocell base station PeNB,information indicating the current target subframe via the X2 interface.Or, the picocell base station PeNB may request the macrocell basestation MeNB or the other picocell base station to notify theinformation indicating the current target subframe.

The PDCCH usage determination unit 122 determines a PDCCH usage of themacrocell base station MeNB in the target subframe determined by thetarget subframe determination unit 121. However, the PDCCH usagedetermination unit 122 may determine the PDCCH usage for all thesubframes. When the PDCCH usage determination unit 122 determines aPDCCH usage for all the subframes, the target subframe determinationunit 121 is unnecessary.

When the self-coverage of the picocell base station PeNB is expanded,the PDCCH usage determination unit 122 determines to decrease the PDCCHusage as compared with the case where the self-coverage is not expanded.When a connection destination is selected not based on RSRP but based ona path loss, or when the RSRP is off set, the self-coverage of thepicocell base station PeNB is expanded. The information on whether theself-coverage is expanded is stored in the storage unit 130 in advanceor is acquired from the macrocell base station MeNB. When an offsetvalue is added to the RSRP of the picocell base station PeNB or when theoffset value is subtracted from the RSRP of the macrocell base stationMeNB, the PDCCH usage determination unit 122 may determine a PDCCH usageaccording to the offset value. Specifically, when the offset value islarge (in other words, the self-coverage of the picocell base stationPeNB is greatly expanded), the PDCCH usage determination unit 122preferably makes a determination so that the PDCCH usage of themacrocell base station MeNB is decreased.

For example, the PDCCH usage determination unit 122 determines thenumber of OFDM symbols in the time axis direction, which are usable asPDCCH in the downlink subframe, in a range of 1 to 3 (or 1 to 4) as aPDCCH usage. Or, the PDCCH usage determination unit 122 determines apercentage of CCE permitting transmission of the downlink controlinformation, among all the CCEs usable as PDCCH in the downlink subframeas a PDCCH usage.

The usage control information generator 123 generates usage controlinformation by associating the target subframe determined by the targetsubframe determination unit 121 with the PDCCH usage determined by thePDCCH usage determination unit 122. Or, the usage control informationgenerator 123 generates usage control information containing informationindicating the PDCCH usage for each downlink subframe determined by thePDCCH usage determination unit 122. The specific configuration exampleof the usage control information is described later.

The X2 interface communication unit 140 transmits the usage controlinformation generated by the usage control information generator 123through the X2 interface. When multiple neighboring base stations arepresent in the neighborhood of the picocell base station PeNB, the X2interface communication unit 140 may transmit the usage controlinformation only to a specific neighboring base station among themultiple neighboring base stations.

(3) Configuration of Macrocell Base Station

Hereinafter, the configuration of the macrocell base station MeNB isdescribed. FIG. 5 is a block diagram showing the configuration of themacrocell base station MeNB.

As shown in FIG. 5, the macrocell base station MeNB has an antenna unit201, a radio communication unit 210, a controller 220, a storage unit230, and an X2 interface communication unit 240. In the firstembodiment, the X2 interface communication unit 240 corresponds to areceiver to receive control information.

The radio communication unit 110 is configured using, for example, aradio frequency (RF) circuit and a base band (BB) circuit, andtransmits/receives a radio signal to/from the radio terminal MUE via theantenna unit 201. Also, the radio communication unit 210 modulates thetransmission signal and demodulates the received signal.

The controller 220 is configured using, for example, a CPU and controlsvarious kinds of functions included in the macrocell base station MeNB.The storage unit 230 is configured using, for example, a memory, andstores various pieces of information to be used for controlling themacrocell base station MeNB and the like. The X2 interface communicationunit 240 performs inter-base station communications with other radiobase stations using the X2 interface.

The controller 220 has a usage control information interpretive unit 221and a PDCCH usage controller 222.

The usage control information interpretive unit 221 interprets usagecontrol information received by the X2 interface communication unit 240and specifies a target subframe and a PDCCH usage thereof. Or, the usagecontrol information interpretive unit 221 interprets usage controlinformation and specifies a PDCCH usage per subframe.

The PDCCH usage controller 222 controls the PDCCH usage for each targetsubframe according to the target subframe and the PDCCH usage thereofwhich are specified by the usage control information interpretive unit221. The PDCCH usage controller 222 controls the number of OFDM symbolsin the time axis direction, which are usable as the PDCCH in thedownlink subframe (in other words, the number of OFDM symbols in thetime axis direction in the control region) within a range of 1 to 3 (or1 to 4) according to the PDCCH usage specified by the usage controlinformation interpretive unit 221. Or, the PDCCH usage controller 222controls the CCE amount (in other words, a coding rate) of PDCCH(control region) according to the PDCCH usage specified by the usagecontrol information interpretive unit 221.

Note that as described above, when the PDCCH of the macrocell basestation MeNB is controlled by another picocell base station, thecontroller 220 may notify the picocell base station PeNB of theinformation indicating the current target subframe by using the X2interface communication unit 240 via the X2 interface. Or, when thecontroller 220 is requested by the picocell base station PeNB to notifythe information indicating the current target subframe, the controller220 may notify the picocell base station PeNB of the informationindicating the current subframe by using the X2 interface communicationunit 240.

(4) Configuration Example of Usage Control Information

Hereinafter, a configuration example of usage control information isdescribed. FIG. 6 is a drawing for illustrating a configuration exampleof usage control information. Note that, in the first embodiment, theusage control information is transmitted at intervals of one radioframe, but it may be transmitted at intervals of one subframe.

As shown in FIG. 6, the usage control information contains a source IDidentifying a source base station of the usage control information, adestination ID identifying a destination base station of the usagecontrol information, and information indicating a PDCCH usage persubframe. The destination ID is contained in the usage controlinformation, so that the usage control information can be transmitted toa specific destination base station.

The information indicating the PDCCH usage for each downlink subframe isconfigured by the following configuration methods.

A first configuration method is that, for example, a PDCCH usage isdesignated for each of 10 subframes contained in one radio frame. Inthis case, the number of OFDM symbols of the PDCCH in the time axisdirection is designated in a range of 1 to 3 (or 1 to 4) for each of the10 subframes contained in the one radio frame. For example, 2-bitinformation is allocated for each subframe, and “00,” “01,” “10,” and“11” are respectively corresponded to 1 symbol, 2 symbols, 3 symbols,and 4 symbols. Or, an allowable CCE amount is designated in a stepwisefashion for each of the 10 subframes contained in the one radio frame.In this case, for example, 2-bit information is allocated for eachsubframe, and “00,” “01,” “10,” and “11” are respectively correspondedto 25%, 50%, 75%, and 100%.

A second configuration method is that, for example, a PDCCH usage iscollectively designated for a target subframe among the 10 subframescontained in the one radio frame. For example, when the number of OFDMsystems in the second and fifth subframes is set to be 2, the PDCCHusage is set to be “01,” and the designated subframe can be configuredby a bitmap of “0100100000.”

(5) Operational Example of Radio Communication System

FIG. 7 is an operational sequence diagram showing an operational exampleof the radio communication system 1A.

At step S11, the target subframe determination unit 121 of the picocellbase station PeNB determines a target subframe in which a PDCCH usage ofthe macrocell base station MeNB is controlled. The PDCCH usagedetermination unit 122 of the picocell base station PeNB determines aPDCCH usage of the macrocell base station MeNB in the target subframedetermined by the target subframe determination unit 121.

At step S12, the usage control information generator 123 of the picocellbase station PeNB generates usage control information by associating thetarget subframe determined by the target subframe determination unit 121with the PDCCH usage determined by the PDCCH usage determination unit122.

At step S13, the X2 interface communication unit 140 of the picocellbase station PeNB transmits the usage control information generated bythe usage control information generator 123 to the macrocell basestation MeNB. The X2 interface communication unit 240 of the macrocellbase station MeNB receives the usage control information.

At step S14, the usage control information interpretive unit 221 of themacrocell base station MeNB interprets the usage control informationreceived by the X2 interface communication unit 240 and specifies atarget subframe and a PDCCH usage thereof.

At step S15, the PDCCH usage controller 222 of the macrocell basestation MeNB controls the PDCCH usage for each target subframe accordingto the target subframe and the PDCCH usage thereof which are specifiedby the usage control information interpretive unit 22.

(6) Effects of First Embodiment

As described above, the picocell base station PeNB can control the PDCCHusage of the macrocell base station with the usage control informationand can reduce the interference of the PDCCH used by the macrocell basestation MeNB, which is received by the PDCCH used by the picocell basestation PeNB. Accordingly, even when the self-coverage of the picocellbase station PeNB is expanded, the radio terminal PUE connecting withthe picocell base station PeNB can properly receive DCI.

Also, the PDCCH usage is controlled according to the usage controlinformation received from the picocell base station PeNB, so that themacrocell base station MeNB can reduce the interference of the PDCCHused by the macrocell base station MeNB, which is given to the PDCCHused by the picocell base station PeNB. Accordingly, even when thepicocell base station PeNB expands the self-coverage, the radio terminalPUE connecting with the picocell base station PeNB can properly receiveDCI.

In the first embodiment, when the self-coverage of the picocell basestation PeNB is expanded, the picocell base station PeNB transmits theusage control information to reduce the PDCCH usage. Accordingly, theradio terminal PUE connecting with the picocell base station PeNB canincrease a possibility that the radio terminal PUE can properly receiveDCI from the picocell base station PeNB even in a case where receivedpower (RSRP) from the macrocell base station MeNB is higher thanreceived power (RSRP) from the picocell base station PeNB.

In the first embodiment, the picocell base station PeNB transmits theusage control information only to the macrocell base station MeNB, sothat a PDCCH usage of another neighboring base station can be preventedfrom being unnecessarily decreased.

Note that although the PDSCH interference between base stations is notdescribed in the above description, the PDSCH can be dealt with existingtechniques such as adaptive modulation control, hybrid automaticretransmission request (HARQ), and cell-to-cell interference control(ICIC).

Second Embodiment

A second embodiment is an embodiment in which a PDCCH interferencebetween macrocell base stations is controlled. Portions different fromthose of the first embodiment are described in the following secondembodiment, and the duplicated description is omitted.

(1) Configuration of Radio Communication System

A schematic configuration of a radio communication system according tothe second embodiment is described. FIG. 8 is a schematic configurationdiagram of a radio communication system 1B according to the secondembodiment.

As shown in FIG. 8, the radio communication system 1B has a macrocellbase station MeNB1, a radio terminal MUE1 connecting with the macrocellbase station MeNB1, a macrocell base station MeNB2 neighboring on themacrocell base station MeNB1, and a radio terminal MUE2 connecting withthe macrocell base station MeNB2 within a cell formed by the macrocellbase station MeNB2.

When the radio terminal MUE1 connected with the macrocell base stationMeNB1 is located near a cell edge, the radio terminal MUE1 is affectedby the interference of the PDCCH used by the macrocell base stationMeNB2 neighboring on the macrocell base station MeNB1. Thus, there is acase where DCI transmitted by the macrocell base station MeNB1 throughthe PDCCH cannot be properly received.

For this reason, in the second embodiment, the macrocell base stationMeNB1 transmits usage control information for controlling the PDCCHusage of the macrocell base station MeNB2 to the macrocell base stationMeNB2 via the X2 interface. Here, as similar to the first embodiment,the PDCCH use amount means, for example, the number of OFDM symbols inthe time axis direction in the control region or a CCE amount in thecontrol region. The PDCCH usage of the macrocell base station MeNB2 isreduced, so that the PDCCH used by the macrocell base station MeNB1 isreduced in interference received from the PDCCH used by the macrocellbase station MeNB2.

In the second embodiment, the block configuration of the macrocell basestation MeNB1 is similar to the block configuration of the picocell basestation PeNB, which is described in the first embodiment, and the blockconfiguration of the macrocell base station MeNB2 is similar to theblock configuration of the macrocell base station MeNB, which isdescribed in the first embodiment.

(2) Operational Example of Radio Communication System

FIG. 9 is an operational sequence diagram showing an operational exampleof the radio communication system 1B according to the second embodiment.

At step S21, the target subframe determination unit 121 of the macrocellbase station MeNB1 determines a target subframe in which a PDCCH usageof the macrocell base station MeNB2 is controlled. The PDCCH usagedetermination unit 122 of the macrocell base station MeNB1 determines aPDCCH usage of the macrocell base station MeNB2 in the target subframedetermined by the target subframe determination unit 121.

At step S22, the usage control information generator 123 of themacrocell base station MeNB1 generates usage control information byassociating the target subframe determined by the target subframedetermination unit 121 with the PDCCH usage determined by the PDCCHusage determination unit 122.

At step S23, the X2 interface communication unit 140 of the macrocellbase station MeNB1 transmits the usage control information generated bythe usage control information generator 123 to the macrocell basestation MeNB2. The X2 interface communication unit 240 of the macrocellbase station MeNB2 receives the usage control information.

At step S24, the usage control information interpretive unit 221 of themacrocell base station MeNB2 interprets the usage control informationreceived by the X2 interface communication unit 240 and specifies atarget subframe and a PDCCH usage thereof.

At step S25, the PDCCH usage controller 222 of the macrocell basestation MeNB2 controls the PDCCH usage for each target subframeaccording to the target subframe and the PDCCH usage thereof which arespecified by the usage control information interpretive unit 221.

(3) Effects of Second Embodiment

As described above, the macrocell base station MeNB1 can control thePDCCH usage of the macrocell base station MeNB2 with the usage controlinformation and can reduce the interference of the PDCCH used by themacrocell base station MeNB, which is received by the PDCCH used by themacrocell base station MeNB2.

Also, the macrocell base station MeNB2 controls the PDCCH usageaccording to the usage control information received from the macrocellbase station MeNB1, and thereby can reduce the interference of the PDCCHused by the macrocell base station MeNB2, which is given to the PDCCHused by the macrocell base station MeNB1.

Third Embodiment

Hereinafter, a third embodiment of the invention is described. Describedin the third embodiment is an interference control technique applied tothe radio communication system 1A by the heterogeneous network same asthat of FIG. 3. Specifically, the description is given in the followingorder of (1) Configuration of Picocell Base Station, (2) Configurationof Macrocell Base Station, (3) Configuration Example of PDCCHNotification Message, (4) Operational Example of Radio CommunicationSystem, and (5) Effects of Third Embodiment. However, portions differentfrom those of the first embodiment are mainly described, and theduplicated description is omitted.

(1) Configuration of Picocell Base Station

FIG. 10 is a block diagram showing the configuration of a picocell basestation PeNB according to the third embodiment. As shown in FIG. 10, apicocell base station PeNB according to the third embodiment isdifferent from that of the first embodiment in the configuration of acontroller 120.

The controller 120 has a deteriorated terminal detector 151, anotification request generator 152, an allocation conditiondetermination unit 153, and a radio resource allocation unit 154.

The deteriorated terminal detector 151 detects a deteriorated radioterminal PUE whose receiving quality is deteriorated due to stronginterference based on reception quality information (such as ameasurement report or CQI) reported from each radio terminal PUEconnected with the picocell base station PeNB. The information on thedetected deteriorated radio terminal PUE is input to the notificationrequest generator 152.

The notification request generator 152 generates a PDCCH notificationrequest message for requesting transmission of a PDCCH notificationmessage in response to the detection of the deteriorated radio terminalPUE. The PDCCH notification message is a message for notifying a PDCCHusage state in the macrocell base station MeNB. The configuration of thePDCCH notification message is described later.

The allocation condition determination unit 153 determines a subframewhose PDCCH and PDSCH are allocated to the deteriorated radio terminalPUE based on the PDCCH notification message received by the X2 interfacecommunication unit 140 from the macrocell base station MeNB. Also, theallocation condition determination unit 153 may determine usages(including a usage rate) of PDCCH resource and PDSCH resource in thedetermined subframe. In the present embodiment, the allocation conditiondetermination unit 153 determines allocation conditions for 10 subframescorresponding to one radio frame.

The radio resource allocation unit 154 allocates PDCCH and PDSCH foreach radio terminal PUE connecting with the picocell base station PeNBfor each subframe according to the allocation conditions (in otherwords, the subframe in which the PDCCH and PDSCH are allocated to thedeteriorated radio terminal PUE and the usages of the PDCCH resource andthe PDSCH resource in the subframe) determined by the allocationcondition determination unit 153.

(2) Configuration of Macrocell Base Station

FIG. 11 is a block diagram showing the configuration of a macrocell basestation MeNB according to the third embodiment. As shown in FIG. 11, amacrocell base station MeNB according to the third embodiment isdifferent from that of the first embodiment in the configuration of acontroller 220.

The controller 220 has a target subframe determination unit 251, anupper limit value determination unit 252, a PDSCH linked-controldetermination unit 253, a PDCCH notification generator 254, and a radioresource allocation unit 255.

The target subframe determination unit 251 interprets the PDCCHnotification request message received by the X2 interface communicationunit 240 from the picocell base station PeNB and determines a limitationtarget subframe in which the PDCCH usage of the macrocell base stationMeNB is limited to an upper limit value or below, among the 10 subframescorresponding to one radio frame.

The upper limit value determination unit 252 interprets the PDCCHnotification request message received by the X2 interface communicationunit 240 and determines an upper limit value of the PDCCH usage in thelimitation target subframe.

The PDSCH linked-control determination unit 253 determines whether tomake linked-control of the PDSCH usage of the macrocell base stationMeNB in the limitation target subframe determined by the target subframedetermination unit 251. The “linked-control” means that the upper limitvalue of the PDCCH usage in the limitation target subframe and the upperlimit value of the PDSCH usage are linked to each other. For example, ifthe upper limit value of the PDCCH usage in the limitation targetsubframe is determined as 10%, the upper limit value of the PDSCH usagein the limitation target subframe is also determined as 10%.

In this manner, the target subframe determination unit 251, the upperlimit value determination unit 252, and the PDSCH linked-controldetermination unit 253 configure the allocation condition determinationunit 250 to determine the allocation conditions for the 10 subframescorresponding to one radio frame.

The PDCCH notification generator 254 generates a PDCCH notificationmessage based on the limitation target subframe determined by the targetsubframe determination unit 251, the upper limit value determined by theupper limit value determination unit 252, and whether to make thelinked-control that is determined by the PDSCH linked-controldetermination unit 253.

The radio resource allocation unit 255 allocates PDCCH and PDSCH to eachradio terminal MUE connecting with the macrocell base station MeNB foreach subframe according to the allocation conditions (in other words,the limitation target subframe, the upper limit value of the PDCCH usagein the limitation target subframe, and the on/off of the linked-controlof the PDSCH usage in the limitation target subframe) determined by theallocation condition determination unit 250.

(3) Configuration Example of PDCCH Notification Message

FIG. 12 is a drawing showing a configuration example of a PDCCHnotification message. As shown in FIG. 12, the PDCCH notificationmessage includes a field F1 storing a source base station ID to identifya source of the message (i.e., the macrocell base station MeNB), a fieldF2 storing a destination base station ID to identify a destination ofthe message (i.e., the picocell base station PeNB), a field F3 storingan information element indicating a limitation target subframe in oneradio frame, a field F4 storing an information element indicating anupper limit value of a PDCCH usage in the limitation target subframe,and a filed F5 storing an information element indicating on/off of thelinked-control of the PDSCH in the limitation target subframe. However,it is also possible to have the configuration such that the field F2storing the destination base station ID is not needed and the message isbroadcasted to all the neighboring base stations.

As similar to the first embodiment, the information element indicatingthe limitation target subframe in one radio frame is configured as, forexample, a bitmap including bits which are respectively associated withthe subframes contained in one radio frame. For example, when the secondand fifth subframes are the limitation target subframes, the second andfifth bits are set as “1” like “0100100000.”

The information element indicating the upper limit value of the PDCCHusage in the limitation target subframe is configured so that “00,”“01,” “10,” and “11” are respectively set as 0%, 20%, 40%, and 60% whenfor example, it is set as 2-bit information.

The information element indicating on/off of the linked-control of thePDSCH in the limitation target subframe is configured as 1-bitinformation such for example as “1” if the linked-control of the PDSCHis “on,” and “0” if the linked-control of the PDSCH is “off.” However,when it is determined in advance that the linked-control of the PDSCH isalways “on” in the case where the upper limit value of the PDCCH usageis 0%, the information element indicating on/off of the linked-controlof the PDSCH is unnecessary.

(4) Operational Example of Radio Communication System

FIG. 13 is an operational sequence diagram showing an operationalexample of the radio communication system 1A according to the thirdembodiment.

At step S301, the deteriorated terminal detector 151 of the picocellbase station PeNB detects a deteriorated radio terminal PUE whosereceiving quality is deteriorated due to strong interference based onreception quality information (such as a measurement report or CQI)reported from each radio terminal PUE connecting with the picocell basestation PeNB. For example, the measurement report contains receivedpower (RSRP) of a reference signal which is received by the radioterminal PUE and information identifying a transmission destination basestation of the reference signal. For this reason, when the RSRP of theradio base station eNB other than the picocell base station PeNB exceedsa threshold, the deteriorated terminal detector 151 detects the radioterminal PUE which is the source of the measurement report as adeteriorated radio terminal PUE receiving a strong interference from theradio base station eNB. When the deteriorated radio terminal PUE isdetected by the deteriorated terminal detector 151, the process proceedsto step S302.

At step S302, the notification request generator 152 of the picocellbase station PeNB generates a PDCCH notification request message forrequesting the transmission of the PDCCH notification message. The PDCCHnotification request message at least contains a source base station IDto identify the picocell base station PeNB. In addition to the sourceID, the PDCCH notification request message may contain informationindicating the number of deteriorated radio terminals PUE detected bythe deteriorated terminal detector 151 and information indicating adegree of interference received by the deteriorated radio terminal PUEdetected by the deteriorated terminal detector 151 (i.e., thedeterioration degree of the receiving quality). Also, the PDCCHnotification request message may contain a destination base station IDto identify the radio base station eNB of the interference source. Theinformation indicating the interference degree and the destination basestation ID can be set based on the reception quality information. ThePDCCH notification request message is assumed below to contain thesource ID, the information indicating the number of deteriorated ratioterminals PUE, and the information indicating the interference degree.

At step S303, the X2 interface communication unit 140 of the picocellbase station PeNB transmits the PDCCH notification request messagegenerated by the notification request generator 152 to each neighboringbase station eNB. The X2 interface communication unit 140 may transmitthe PDCCH notification request message only to the neighboring basestation eNB shown by the destination ID contained in the PDCCHnotification request message.

The X2 interface communication unit 240 of the macrocell base stationMeNB receives the PDCCH notification request message.

At step S304, the target subframe determination unit 251 of themacrocell base station MeNB interprets the PDCCH notification requestmessage which is received by the X2 interface communication unit 240 anddetermines a limitation target subframe in which the PDCCH usage is tobe limited to an upper limit value or below among the 10 subframescorresponding to one radio frame. The target subframe determination unit251 may determine the limitation target subframe according to theinformation indicating the number of the deteriorated radio terminalsPUE, which is contained in the PDCCH notification request message. Forexample, the target subframe determination unit 251 increases the numberof limitation target subframes as the number of the deteriorated radioterminals PUE is larger, and decreases the number of limitation targetsubframes as the number of the deteriorated radio terminals PUE issmaller. However, when the PDCCH notification request message is beingreceived from another picocell base station PeNBx (not shown), thetarget subframe determination unit 251 determines a limitation targetsubframe in consideration of the contents of the PDCCH notificationrequest message from another picocell base station PeNBx.

In addition, at step S304, the upper limit value determination unit 252of the macrocell base station MeNB interprets the PDCCH notificationrequest message received by the X2 interface communication unit 240 anddetermines an upper limit value of the PDCCH usage in the limitationtarget subframe. The upper limit value determination unit 252 maydetermines an upper limit value according to the information indicatingthe interference degree, which is contained in the PDCCH notificationrequest message. For example, the upper limit value determination unit252 decreases the upper limit value as the interference degree is largerand increases the upper limit value as the interference degree issmaller. However, when the PDCCH notification request message is beingreceived from another picocell base station PeNBx, the upper limit valuedetermination unit 252 determines an upper limit value in considerationof the contents of the PDCCH notification request message from anotherpicocell base station PeNBx.

At step S305, the PDSCH linked-control determination unit 253 of themacrocell base station MeNB determines whether to make thelinked-control of the PDSCH usage in the limitation target subframedetermined by the target subframe determination unit 251. The PDSCHlinked-control determination unit 253 uses the upper limit value of thePDCCH usage, which is determined by the upper limit value determinationunit 252 as a reference for determining the on/off of thelinked-control. When the upper limit value of the PDCCH usage is low,PDSCH can be allocated to only a small number of radio terminals UE inthe limitation target subframe, which decreases a scheduling gain.Accordingly, when the upper limit value of the PDCCH usage is lower thana predetermined threshold, the PDSCH linked-control determination unit253 determines to make the linked-control of the PDSCH usage in thelimitation target subframe for decreasing the PDSCH allocation in thelimitation target subframe.

At step S306, the PDCCH notification generator 254 generates a PDCCHnotification message based on the target subframe determined by thetarget subframe determination unit 251, the upper limit value determinedby the upper limit value determination unit 252, and the on/off of thelinked-control determined by the PDSCH linked-control determination unit253. In addition to the source base station ID to identify the macrocellbase station MeNB, the PDCCH notification message contains (a) aninformation element indicating the limitation target subframe in oneradio frame, (b) an information element indicating the upper limit valueof the PDCCH usage in the limitation target subframe, and (c) aninformation element indicating on/off of the linked-control of the PDSCHin the limitation target subframe. However, like a case where thelinked-control of the PDSCH is always “on” when the upper limit value ofthe PDCCH usage shows 0%, the information element of (c) may beindicated by using the information element of (b). Also, the PDCCHnotification message may contain a destination base station ID.Specifically, the source base station ID of the PDCCH notificationrequest message can be set as a destination base station ID of the PDCCHnotification message. Or, the PDCCH notification message may bebroadcasted to all the neighboring base stations eNB around themacrocell base station MeNB. In doing so, all the neighboring basestations eNB having received the PDCCH notification message can know thePDCCH usage state of the macrocell base station MeNB and can utilize itfor the PDCCH allocation in each neighboring base station eNB.

At step S307, the X2 interface communication unit 240 of the macrocellbase station MeNB transmits the PDCCH notification message generated bythe PDCCH notification generator 254.

The X2 interface communication unit 140 of the picocell base stationPeNB receives the PDCCH notification message.

At step S308, the allocation condition determination unit 153 of thepicocell base station PeNB determines a subframe whose PDCCH and PDSCHare allocated to the deteriorated radio terminal PUE based on (a) theinformation element indicating the limitation target subframe in oneradio frame, (b) the information element indicating the upper limitvalue of the PDCCH usage in the limitation target subframe, and (c) theinformation element indicating on/off of the linked-control of the PDSCHin the limitation target subframe, which are contained in the PDCCHnotification message received by the X2 interface communication unit140. The allocation condition determination unit 153 determines thelimitation target subframe shown by the information element of (a) as asubframe whose PDCCH and PDSCH are allocated to the deteriorated radioterminal PUE. In addition, the allocation condition determination unit153 may determine to increase the PDCCH resource usage in the limitationtarget subframe as the upper limit value indicated by the informationelement of (b) becomes lower. In addition, the allocation conditiondetermination unit 153 may determine to increase the PDSCH resourceusage in the limitation target subframe when the information element of(c) indicates that the linked-control of the PDSCH is “on.”

Note that the processes at step S304 to S307 may be repeatedly executedfor each radio frame (10 subframes) until the picocell base station PeNBtransmits a transmission stop request for the PDCCH notification messageto the macrocell base station MeNB. In this case, the picocell basestation PeNB preferably transmits the transmission stop request for thePDCCH notification message to the macrocell base station MeNB when thedeteriorated radio terminal PUE becomes undetectable.

Or, the picocell base station PeNB may transmit information indicatingthe number of repeated executions of the processes at step S304 to S307with the information contained in the PDCCH notification request. Inthis case, the macrocell base station MeNB repeatedly executes theprocesses at step S304 to S307 by the number of times according to theinformation indicating the number of repeated executions contained inthe PDCCH notification request.

Note that, although in the present operational sequence, the PDCCHnotification message is transmitted from the macrocell base station MeNBto the picocell base station PeNB, the PDCCH notification message may betransmitted from the picocell base station MeNB to the macrocell basestation PeNB.

(5) Effects of Third Embodiment

The third embodiment can suppress the PDCCH interference between theradio base stations as similar to the first embodiment. Also, the PDSCHusage is linked to the PDCCH usage, so that the PDSCH interferencebetween radio base stations can be also suppressed.

Note that the third embodiment describes an example of an interferencecontrol technique in the heterogeneous network. However, theinterference control technique according to the third embodiment isapplicable to the PDCCH interference between macrocell base stations asdescribed in the second embodiment.

Fourth Embodiment

Hereinafter, a fourth embodiment of the invention is described. Thefourth embodiment is an embodiment in which the first embodiment and thethird embodiment are combined. A macrocell base station MeNB accordingto the fourth embodiment is configured as similar as that of the thirdembodiment. In the fourth embodiment, the description is given in thefollowing order of (1) Configuration of Picocell Base Station, (2)Operational Example of Radio Communication System, (3) and Effects ofFourth Embodiment. Also, portions different from those of the first andthird embodiments are mainly described, and the duplicated descriptionis omitted.

(1) Configuration of Picocell Base Station

FIG. 14 is a block diagram showing the configuration of a picocell basestation PeNB according to the fourth embodiment. As shown in FIG. 14, apicocell base station PeNB according to the fourth embodiment isdifferent from that of the first embodiment in the configuration of acontroller 120.

The controller 120 has a deteriorated terminal detector 151, a targetsubframe determination unit 162, an upper limit value determination unit163, a limitation request generator 164, an allocation conditiondetermination unit 153, and a radio resource allocation unit 154. Thedeteriorated terminal detector 151, the allocation conditiondetermination unit 153, and the radio resource allocation unit 154 haveconfigurations same as those of the third embodiment.

The target subframe determination unit 162 determines a limitationtarget subframe in which a PDCCH usage of the macrocell base stationMeNB should be limited to the upper limit value or below among 10subframes corresponding to one radio frame.

The upper limit value determination unit 163 determines the upper limitvalue of the PDCCH usage of the macrocell base station MeNB in thelimitation target subframe determined by the target subframedetermination unit 162.

The limitation request generator 164 generates a PDCCH limitationrequest message based on the target subframe determined by the targetsubframe determination unit 162 and the upper limit value determined bythe upper limit value determination unit 163. The PDCCH limitationrequest message contains (a) an information element indicating alimitation target subframe in one radio frame and (b) an informationelement indicating an upper limit value of the PDCCH usage of themacrocell base station MeNB in the limitation target subframe, inaddition to a source base station ID to identify the picocell basestation PeNB and a destination base station ID to identify theinterference source radio base station eNB (the macrocell base stationMeNB).

The PDCCH limitation request message can have a configuration in which afield storing the information element indicating on/off of thelinked-control of the PDSCH is excluded from a message format of thePDCCH notification message described in the third embodiment.

(2) Operational Example of Radio Communication System

FIG. 15 is an operational sequence diagram showing an operationalexample of a radio communication system 1A according to the fourthembodiment.

At step S401, the deteriorated terminal detector 151 of the picocellbase station PeNB detects a deteriorated radio terminal PUE whosereceiving quality is deteriorated due to a strong interference based onreception quality information (such as a measurement report or CQI)reported from each radio terminal PUE connecting with the picocell basestation PeNB. When the deteriorated radio terminal PUE is detected bythe deteriorated terminal detector 151, the process proceeds to stepS402.

At step S402, the target subframe determination unit 162 of the picocellbase station PeNB determines a limitation target subframe in which aPDCCH usage of the macrocell base station MeNB should be limited to theupper limit value or below among the 10 subframes corresponding to oneradio frame. The target subframe determination unit 162 may determine alimitation target subframe according to the number of deteriorated radioterminals PUE which are detected by the deteriorated terminal detector151. For example, the target subframe determination unit 162 increasesthe number of limitation target subframes as the number of thedeteriorated radio terminals PUE becomes larger, and decreases thenumber of limitation target subframes as the number of the deterioratedradio terminals PUE becomes smaller.

At step S403, the upper limit value determination unit 163 of thepicocell base station PeNB determines the upper limit value of the PDCCHusage of the macrocell base station MeNB in the limitation targetsubframe determined by the target subframe determination unit 162. Theupper limit value determination unit 163 may determine an upper limitvalue according to an interference degree (i.e., the degree of receivingquality deterioration) with respect to the deteriorated radio terminalPUE detected by the deteriorated terminal detector 151. For example, theupper limit value determination unit 163 decreases the upper limit valueas the interference degree is larger and increases the upper limit valueas the interference degree is smaller.

At step S403, the limitation request generator 164 of the picocell basestation PeNB generates a PDCCH limitation request message based on thetarget subframe determined by the target subframe determination unit 162and the upper limit value determined by the upper limit valuedetermination unit 163. The PDCCH limitation request message contains(a) an information element indicating a limitation target subframe inone radio frame and (b) an information element indicating an upper limitvalue of the PDCCH usage in the limitation target subframe, in additionto a source base station ID to identify the picocell base station PeNBand a destination base station ID to identify the interference sourceradio base station eNB (the macrocell base station MeNB).

At step S404, the X2 interface communication unit 140 of the picocellbase station PeNB transmits the PDCCH limitation request messagegenerated by the limitation request generator 164 to the interferencesource radio base station eNB (the macrocell base station MeNB).

The X2 interface communication unit 240 of the macrocell base stationMeNB receives the PDCCH limitation request message.

At step S405, the target subframe determination unit 251 of themacrocell base station MeNB determines a limitation target subframe inwhich the PDCCH usage should be limited to the upper limit value orbelow among the 10 subframes corresponding to one radio frame, accordingto the information element of (a) contained in the PDCCH limitationrequest message received by the X2 interface communication unit 240.When the PDCCH limitation request message is being received from anotherpicocell base station PeNBx, the target subframe determination unit 251determines a limitation target subframe in consideration of the contentsof the PDCCH limitation request message from another picocell basestation PeNBx.

In addition, at step S405, the upper limit value determination unit 252of the macrocell base station MeNB determines the above-described upperlimit value according to the information element of (b) contained in thePDCCH limitation request message received by the X2 interfacecommunication unit 240. When the PDCCH limitation request message isalso being received from another picocell base station PeNBx, the upperlimit value determination unit 252 determines an upper limit value inconsideration of the contents of the PDCCH limitation request messagefrom another picocell base station PeNB.

At step S406, the PDSCH linked-control determination unit 253 of themacrocell base station MeNB determines whether to make thelinked-control of the PDSCH usage in the limitation target subframewhose PDCCH usage is determined to be limited to the upper limit valueor below. The PDSCH linked-control determination unit 253 uses the upperlimit value of the PDCCH usage, which is determined by the upper limitvalue determination unit 252 as a reference for determining the on/offof the linked-control. When the upper limit value of the PDCCH usage islower than a predetermined threshold, the PDSCH linked-controldetermination unit 253 determines to make the linked-control of thePDSCH usage in the limitation target subframe for decreasing the PDSCHallocation in the limitation target subframe.

At step S407, the PDCCH notification generator 254 generates a PDCCHnotification message based on a target subframe determined by the targetsubframe determination unit 251, the upper limit value determined by theupper limit value determination unit 252, and the on/off of thelinked-control determined by the PDSCH linked-control determination unit253. The specific creation method is similar to step S306 in the thirdembodiment.

At step S408, the X2 interface communication unit 240 of the macrocellbase station MeNB transmits the PDCCH notification message generated bythe PDCCH notification generator 254.

The X2 interface communication unit 140 of the picocell base stationPeNB receives the PDCCH notification message.

At step S409, the allocation condition determination unit 153 of thepicocell base station PeNB determines a subframe whose PDCCH and PDSCHare allocated to the deteriorated radio terminal PUE based on (a) theinformation element indicating the limitation target subframe in oneradio frame, (b) the information element indicating the upper limitvalue of the PDCCH usage in the limitation target subframe, and (c) theinformation element indicating on/off of the linked-control of the PDSCHin the limitation target subframe, which are contained in the PDCCHnotification message received by the X2 interface communication unit140. The specific determination method is similar to step S308 in thethird embodiment.

Note that, although in the present operational sequence, the PDCCHlimitation request message is transmitted from the picocell base stationMeNB to the macrocell base station PeNB, the PDCCH limitation requestmessage may be transmitted from the macrocell base station MeNB to thepicocell base station PeNB.

(3) Effects of Fourth Embodiment

The fourth embodiment can suppress the PDCCH interference between theradio base stations as similar to the first embodiment. Also, the PDSCHusage is linked to the PDCCH usage, so that the PDSCH interferencebetween radio base stations can be suppressed. Note that the fourthembodiment describes the example of the interference control techniquein the heterogeneous network. However, the interference controltechnique according to the fourth embodiment is applicable to the PDCCHinterference between macrocell base stations as described in the secondembodiment.

Fifth Embodiment

Hereinafter, a fifth embodiment of the invention is described. The fifthembodiment is an embodiment in which the third embodiment is modified. Amacrocell base station MeNB according to the fifth embodiment isconfigured as similar as that of the third embodiment. In the fifthembodiment, the description is given in the following order of (1)Configuration of Picocell Base Station, (2) Operational Example of RadioCommunication System, (3) and Effects of Fifth Embodiment. Also,portions different from those of the first, third and fifth embodimentsare mainly described, and the duplicated description is omitted.

(1) Configuration of Picocell Base Station

FIG. 16 is a block diagram showing the configuration of a picocell basestation PeNB according to a fifth embodiment. As shown in FIG. 16, apicocell base station PeNB according to the fifth embodiment isdifferent from that of the first embodiment in the configuration of acontroller 120.

The controller 120 has a deteriorated terminal detector 151, a targetsubframe determination unit 172, an upper limit value determination unit173, a PDSCH linked-control determination unit 174, a PDCCH notificationgenerator 175, an allocation condition determination unit 153, and aradio resource allocation unit 154.

The deteriorated terminal detector 151, the allocation conditiondetermination unit 153, and the radio resource allocation unit 154 haveconfigurations same as those of the third embodiment.

The target subframe determination unit 172 determines a limitationtarget subframe in which a PDCCH usage of the picocell base station PeNBis to be limited to an upper limit value or below among 10 subframescorresponding to one radio frame.

The upper limit value determination unit 173 determines the upper limitvalue of the PDCCH usage in the limitation target subframe.

The PDSCH linked-control determination unit 174 determines whether tomake the linked-control of the PDSCH usage of the picocell base stationPeNB in the limitation target subframe determined by the target subframedetermination unit 172.

The PDCCH notification generator 175 generates a PDCCH notificationmessage based on the limitation target subframe determined by the targetsubframe determination unit 172, the upper limit value determined by theupper limit value determination unit 173, and the on/off of thelinked-control determined by the PDSCH linked-control determination unit174.

(2) Operational Example of Radio Communication System

FIG. 17 is an operational sequence diagram showing an operationalexample of a radio communication system 1A according to the fifthembodiment.

At step S501, the deteriorated terminal detector 151 of the picocellbase station PeNB detects a deteriorated radio terminal PUE whosereceiving quality is deteriorated due to a strong interference based onreception quality information (such as a measurement report or CQI)reported from each radio terminal PUE connecting with the picocell basestation PeNB. When the deteriorated radio terminal PUE is detected bythe deteriorated terminal detector 151, the process proceeds to stepS502.

At step S502, the target subframe determination unit 172 of the picocellbase station PeNB determines a limitation target subframe in which thePDCCH usage of the picocell base station PeNB should be limited to theupper limit value or below among 10 subframes corresponding to one radioframe based on the information on the deteriorated radio terminal PUE.The target subframe determination unit 172 may determine a limitationtarget subframe according to the number of deteriorated radio terminalsPUE. For example, the target subframe determination unit 172 decreasesthe number of limitation target subframes as the number of thedeteriorated radio terminals PUE is larger, and increases the number oflimitation target subframes as the number of the deteriorated radioterminals PUE is smaller.

Also, at step S503, the upper limit value determination unit 173 of thepicocell base station PeNB determines an upper limit value of the PDCCHusage of the picocell base station PeNB in the limitation targetsubframe based on the information on the deteriorated radio terminalPUE. The upper limit value determination unit 173 may determine an upperlimit value of the deteriorated radio terminal PUE according to aninterference degree. For example, the upper limit value determinationunit 173 decreases the upper limit value as the interference degreebecomes larger and increases the upper limit value as the interferencedegree becomes smaller.

At step S503, the PDSCH linked-control determination unit 174 of thepicocell base station PeNB determines whether to make the linked-controlof the PDSCH usage of the picocell base station PeNB in the limitationtarget subframe determined by the target subframe determination unit172. The PDSCH linked-control determination unit 174 uses the upperlimit value of the PDCCH usage, which is determined by the upper limitvalue determination unit 173, as a reference for determining the on/offof the linked-control. When the upper limit value of the PDCCH usage islow, the PDSCH can be allocated to only a small number of radioterminals PUE in the limitation target subframe, which decreases ascheduling gain. Accordingly, when the upper limit value of the PDCCHusage of the picocell base station PeNB is lower than the predeterminedthreshold, the PDSCH linked-control determination unit 174 determines tomake the linked-control of the PDSCH usage of the picocell base stationPeNB in the limitation target subframe for decreasing the PDSCHallocation of the picocell base station PeNB in the limitation targetsubframe.

At step S504, the PDCCH notification generator 175 generates a PDCCHnotification message based on the target subframe determined by thetarget subframe determination unit 172, the upper limit value determinedby the upper limit value determination unit 173, and the on/off of thelinked-control determined by the PDSCH linked-control determination unit174. The PDCCH notification message contains (a) an information elementindicating the limitation target subframe in one radio frame, (b) aninformation element indicating the upper limit value of the PDCCH usagein the limitation target subframe, and (c) an information elementindicating on/off of the linked-control of the PDSCH in the limitationtarget subframe, in addition to a source base station ID to identify thepicocell base station PeNB. However, like a case where thelinked-control of the PDSCH is always on when the upper limit value ofthe PDCCH usage is 0%, the information element of (c) may be indicatedby using the information element of (b). Also, the PDCCH notificationmessage may contain a destination base station ID. Instead, the PDCCHnotification message may be broadcasted to all the neighboring basestations eNB around the picocell base station MeNB. Accordingly, all theneighboring base stations eNB which have received the PDCCH notificationmessage can know the PDCCH usage state of the picocell base stationPeNB.

At step S505, the X2 interface communication unit 140 of the picocellbase station PeNB transmits the PDCCH notification message generated bythe PDCCH notification generator 175.

The X2 interface communication unit 240 of the macrocell base stationMeNB receives the PDCCH notification message.

At step S506, the target subframe determination unit 251 of themacrocell base station MeNB interprets the PDCCH notification messagewhich is received by the X2 interface communication unit 240 anddetermines a limitation target subframe in which the PDCCH usage shouldbe to the upper limit value or below among the 10 subframescorresponding to one radio frame. The target subframe determination unit251 may determines a limitation target subframe according to (a) aninformation element indicating a limitation target subframe in one radioframe and (b) an information element indicating an upper limit value ofthe PDCCH usage in the limitation target subframe, included in the PDCCHnotification message. For example, the target subframe determinationunit 251 recognizes that a subframe whose PDCCH usage is not limited tothe upper limit value or below by the picocell base station PeNB isallocated to a cell edge terminal (a deteriorated radio terminal PUE) ofthe picocell base station PeNB, and determines the subframe as alimitation target subframe whose PDCCH usage of its own base station(macrocell base station MeNB) should be limited to the upper limit valueor below.

In addition, at step S506, the upper limit value determination unit 252of the macrocell base station MeNB interprets the PDCCH notificationmessage received by the X2 interface communication unit 240 anddetermines an upper limit value of the PDCCH usage in the limitationtarget subframe. The upper limit value determination unit 252 maydetermine its own upper limit value according to (b) the informationindicating the upper limit value of the PDCCH usage in the limitationtarget subframe contained in the PDCCH notification message. Forexample, the upper limit value determination unit 252 determines theupper limit value of the PDCCH usage of its own base station (of themacrocell base station MeNB) to be the upper limit value of the PDCCHusage of the picocell base station PeNB. Or, the upper limit valuedetermination unit 252 may determine whether or not a target is apicocell base station PeNB and determine the upper limit value of itsown base station to be an upper limit value which is predetermined forthe picocell base station PeNB.

At step S507, the PDSCH linked-control determination unit 253 of themacrocell base station MeNB determines whether to make thelinked-control of the PDSCH usage in the limitation target subframedetermined by the target subframe determination unit 251. The PDSCHlinked-control determination unit 253 uses the upper limit value of thePDCCH usage, which is determined by the upper limit value determinationunit 252, as a reference for determining the on/off of thelinked-control. When the upper limit value of the PDCCH usage is low,the PDSCH can be allocated to only a small number of radio terminals UEin the limitation target subframe becomes smaller, which decreases ascheduling gain. Accordingly, when the upper limit value of the PDCCHusage is lower than the predetermined threshold, the PDSCHlinked-control determination unit 253 determines to make thelinked-control of the PDSCH usage in the limitation target subframe fordecreasing the PDSCH allocation in the limitation target subframe.

At step S508, the PDCCH notification generator 254 generates a PDCCHnotification message based on the target subframe determined by thetarget subframe determination unit 251, the upper limit value determinedby the upper limit value determination unit 252, and the on/off of thelinked-control determined by the PDSCH linked-control determination unit253. The PDCCH notification message contains (a) an information elementindicating the limitation target subframe of the macrocell base stationMeNB in one radio frame, (b) an information element indicating its ownupper limit value of the PDCCH usage in the limitation target subframe,and (c) an information element indicating on/off of the linked-controlof the PDSCH of the macrocell base station MeNB in the limitation targetsubframe, in addition to a source base station ID to identify themacrocell base station MeNB. However, like a case where thelinked-control of the PDSCH is always on when the upper limit value ofthe PDCCH usage is 0%, the information element of (c) may be indicatedby us ing the information element of (b). Also, the PDCCH notificationmessage may contain a destination base station ID. Specifically, thesource base station ID of the PDCCH notification message received by themacrocell base station MeNB can be set as a destination base station IDof the PDCCH notification message to be transmitted by the macrocellbase station MeNB. Or, the PDCCH notification message may be broadcastedto all the neighboring base stations eNB around the macrocell basestation MeNB.

At step S509, the X2 interface communication unit 240 of the macrocellbase station MeNB transmits the PDCCH notification message generated bythe PDCCH notification generator 254.

The X2 interface communication unit 140 of the picocell base stationPeNB receives the PDCCH notification message.

At step S510, the allocation condition determination unit 153 of thepicocell base station PeNB determines a subframe whose PDCCH and PDSCHare allocated to the deteriorated radio terminal PUE based on (a) theinformation element indicating the limitation target subframe in oneradio frame, (b) the information element indicating the upper limitvalue of the PDCCH usage in the limitation target subframe, and (c) theinformation element indicating on/off of the linked-control of the PDSCHin the limitation target subframe, which are contained in the PDCCHnotification message received by the X2 interface communication unit140. The allocation condition determination unit 153 determines alimitation target subframe shown by the information element of (a) as asubframe whose PDCCH and PDSCH are allocated to the deteriorated radioterminal PUE. In addition, the allocation condition determination unit153 may determine to increase the PDCCH resource usage in the limitationtarget subframe as the upper limit value indicated by the informationelement of the allocation condition determination unit 153 becomeslower. In addition, the allocation condition determination unit 153 maydetermine to increase the PDCCH resource usage in the limitation targetsubframe when the information element of (c) indicates that thelinked-control of the PDSCH is “on.”

(3) Effects of Fifth Embodiment

The fifth embodiment can suppress the PDCCH interference between theradio base stations as similar to the first embodiment. Also, the PDSCHusage is linked to the PDCCH usage, so that the PDSCH interferencebetween radio base stations can be also suppressed. Note that the fifthembodiment describes an example of the interference control technique inthe heterogeneous network. However, the interference control techniqueaccording to the fifth embodiment is applicable to the PDCCHinterference between macrocell base stations as described in the secondembodiment.

Sixth Embodiment

Although each of the above-described embodiments mainly describes asuppression of interference which is received by the PDCCH of thepicocell base station PeNB from the PDCCH of the macrocell base stationMeNB, the PDCCH of the picocell base station PeNB may receive aninterference not only from the PDCCH of the macrocell base station MeNBbut also the PDSCH of the macrocell base station MeNB. A sixthembodiment is an embodiment which also considers an interference whichis received by a PDCCH of a picocell base station PeNB from a PDSCH of amacrocell base station MeNB.

FIG. 18 is a drawing for illustrating an interference state of the PDCCHof the picocell base station PeNB. As shown in FIG. 18, when a PDCCHdomain (a control region) of the picocell base station PeNB is for 3symbols from the head of subframes and a PDCCH domain of the macrocellbase station MeNB is for 2 symbols from the head of subframes, the PDCCHdomain for the third symbol of the picocell base station PeNB receivesan interference from a PDSCH domain (a data region) of the macrocellbase station MeNB. In this case, to reduce the interference received bythe PDCCH of the picocell base station PeNB, a PDSCH usage rate of themacrocell base station MeNB has to be considered.

In the case where a PDCCH limitation is requested from the picocell basestation PeNB to the macrocell base station MeNB as in above-describedthird and fourth embodiments, the PDSCH usage rate of the macrocell basestation MeNB can be considered by adding the following changes. As shownin FIG. 19, in the present modification, the picocell base station PeNBincludes information indicating the number of symbols within a timerange of the PDCCH domain (the control region) of the picocell basestation PeNB (the PDCCH symbol number information) and informationindicating a radio resource usage rate to be used as an upper limit bythe macrocell base station MeNB within the time range (the upper limitvalue information). The macrocell base station MeNB having received thismessage controls the usage rate of radio resource (PDCCH resource andPDSCH resource) corresponding to the time range of the PDCCH domain ofthe picocell base station PeNB in the limitation target subframedesignated by this message.

In addition, in the case where PDCCH is notified from the picocell basestation PeNB to the macrocell base station MeNB as in theabove-described fifth embodiment, the PDSCH usage rate of the macrocellbase station MeNB can be also considered by adding the followingchanges. As shown in FIG. 19, in the present modification, the picocellbase station PeNB includes information indicating the number of symbolswithin a time range of the PDCCH domain (the control region) of thepicocell base station PeNB (PDCCH symbol number information) andinformation indicating a radio resource usage rate to be used as anupper limit by the picocell base station PeNB within the time range (theupper limit value information). The macrocell base station MeNB havingreceived this message controls the usage rate of radio resource (PDCCHresource and PDSCH resource) corresponding to the time range of thePDCCH domain of the picocell base station PeNB in a subframe other thanthe limitation target subframe designated by this message.

Note that the PDCCH notification message from the macrocell base stationMeNB to the picocell base station PeNB preferably can notify whether themacrocell base station MeNB limits the radio resource usage rate for thenumber of symbols (the time range) according to the notification fromthe picocell base station PeNB. Accordingly, the macrocell base stationMeNB may transmit information indicating the number of symbols withinthe time range in which the radio resource usage rate is limited (thelimitation symbol number information) to the picocell base station PeNBwith the information being contained in the PDCCH notification message,in addition to the information indicating the number of symbols withinthe time range of the PDCCH domain (the control region) of the macrocellbase station MeNB (the PDCCH symbol number information) and theinformation indicating the radio resource usage rate to be used as anupper limit by the macrocell base station MeNB within the time range(the upper limit value information). In the example of FIG. 19, themacrocell base station MeNB notifies “2” symbols as the PDCCH symbolnumber information and “3” symbols as the limitation symbol numberinformation.

Similarly, the limitation symbol number information may be employed asthe PDCCH notification message from the picocell base station PeNB tothe macrocell base station MeNB. In the example of FIG. 19, the picocellbase station PeNB notifies “3” symbols as both the PDCCH symbol numberinformation and the limitation symbol number information.

Seventh Embodiment

Hereinafter, a seventh embodiment of the invention is described.Described in the seventh embodiment is an interference control techniqueapplied to a radio communication system of a heterogeneous network sameas that of FIG. 3.

In the above-described embodiments, the picocell base station PeNB andthe macrocell base station MeNB each determine whether its own PDCCHusage (PDCCH usage rate) is linked to its own PDSCH usage (PDSCH usagerate) in a limitation target subframe of the base station itself, andnotify each other of information indicating on/off of the linked-controlof the PDSCH.

On the other hand, in the present embodiment, in place of theinformation indicating on/off of the linked-control of the PDSCH,information referred to as RNTP (Relative Narrowband Tx Power) is used.The RNTP is information indicating whether downlink transmission poweris limited to a RNTP threshold or below for each resource block (RB).The RNTP contains a string of bits associated with respective RBs andthe RNTP threshold. For example, when a RB whose transmission power islimited the RNTP threshold or below is “0” and a RB whose transmissionpower is not limited to the RNTP threshold or below is “1,” the bitstring associated with RBs is configured like “10100 . . . ” with eachRB represented by the position of a bit.

The description of the seventh embodiment is given in the order of (1)Summary of Seventh Embodiment and (2) Details of Seventh Embodiment.Note that portions different from those of the first embodiment to thesixth embodiment are described, and the duplicated description isomitted.

(1) Summary of Seventh Embodiment

A first characteristic according to a seventh embodiment is that a radiobase station includes a transmitter configured to transmit transmissionpower information to a neighboring base station through inter-basestation communication when the radio base station performs downlinkcommunication with a radio terminal by using a communication frameconfiguration in which subframes are arranged in a time direction aredivided by time into a control region and a time domain. Specifically,for a limitation target subframe where a downlink control channel usageor a downlink control channel usage rate is limited, when the radio basestation limits the downlink transmission power for all the frequencybands in the data region to a threshold or below, the transmitter sendsthe transmission power information indicating that the downlinktransmission power in all the frequency bands in the data region islimited to the threshold or below in the limitation target subframe.

Also, a second characteristic of the radio communication systemaccording to the seventh embodiment is that the transmitter transmitsinformation indicating the threshold together with the transmissionpower information in the first characteristic.

Furthermore, a third characteristic of the radio communication systemaccording to the seventh embodiment is that the radio base stationincludes a transmission power controller configured to limit downlinktransmission power so to the threshold or below only for part of thefrequency bands in the data region in a subframe other than thelimitation target subframe in the first characteristic.

In the first to third characteristics, for example, the downlink controlchannel means a PDCCH, the frequency band means a resource block (RB),the threshold means an RNTP threshold, the transmission powerinformation means a bit string associated with RBs, and the inter-basestation communication means communications via an X2 interface.

(2) Details of Seventh Embodiment

Hereinafter, the details of the seventh embodiment are described in thefollowing order of (2.1) Configuration of Macrocell Base Station, (2.2)Configuration of Picocell Base Station, and (2.3) Operational Example ofRadio Communication System. The configuration and operation based on thefifth embodiment is described herein but a method of the sixthembodiment may be incorporated.

(2.1) Configuration of Macrocell Base Station

FIG. 20 is a block diagram showing the configuration of a macrocell basestation MeNB according to a seventh embodiment.

As shown in FIG. 20, a controller 220 of a macrocell base station MeNBaccording to the seventh embodiment has a target subframe determinationunit 218, an upper limit value determination unit 282, a PDSCHlinked-control determination unit 283, a notification generator 284, atransmission power controller 285, and a radio resource allocation unit286.

The target subframe determination unit 281 determines a limitationtarget subframe in which a PDCCH usage of the macrocell base stationMeNB is limited to the upper limit value or below, among multiplesubframes to be used in the future. Note that the term “usage” containsthe concepts of “usage rate” and “transmission power.” Specifically, theusage (in other words, energy) is determined by a product of the usagerate and the transmission power.

The upper limit value determination unit 282 determines an upper limitvalue of the PDCCH usage in the limitation target subframe of themacrocell base station MeNB. Also, in the present embodiment, the upperlimit value determination unit 282 determines a RNTP threshold accordingto the upper limit value of the determined PDCCH usage. Note that theupper limit value determination unit 282 may determine an RNTP thresholdof the macrocell base station MeNB based on information different fromthe information on the upper limit value of the determined PDCCH usage.

The PDSCH linked-control determination unit 283 determines whether tomake linked-control of the PDSCH usage of the macrocell base stationMeNB in the limitation target subframe determined by the target subframedetermination unit 281. When the linked-control of the PDSCH usage ison, the downlink transmission power of all the RBs in the limitationtarget subframe of the macrocell base station MeNB is limited to theRNTP threshold or below. Also, for a subframe other than the limitationtarget subframe of the macrocell base station MeNB (hereinafter,referred to as a non-limitation target subframe), the downlinktransmission power of only part of RBs in the data region is limited tothe RNTP threshold or below.

The notification generator 284 generates a notification messagecontaining the information indicating the limitation target subframedetermined by the target subframe determination unit 281, theinformation indicating the upper limit value determined by the upperlimit value determination unit 282 and the RNTP threshold, and the RNTPinformation (the bit string associated with each RB).

When the linked-control of the PDSCH usage is determined to be on, theRNTP information for the limitation target subframe of the macrocellbase station MeNB indicates that the downlink transmission power of allthe RBs in the data region of the limitation target subframe of themacrocell base station MeNB is limited to the RNTP threshold or below.On the other hand, when the linked-control of the PDSCH usage isdetermined to be off, the RNTP information for each of the limitationtarget subframe and the non-limitation target subframe of the macrocellbase station MeNB indicates that the downlink transmission power of onlypart of the RBs in the data region is limited to the RNTP threshold orbelow.

Note that the notification message may further include an ID showing asource base station (or a source cell) and an ID showing a transmissiondestination base station (or a transmission destination cell).

The notification generator 284 controls the X2 interface communicationunit 240 so that the generated notification message is transmitted tothe picocell base station PeNB. The X2 interface communication unit 240(and the notification generator 284) configures a transmitter totransmit control information in the present embodiment.

The radio resource allocation unit 285 allocates PDCCH and PDSCH withrespect to each radio terminal MUE connecting with the macrocell basestation MeNB for each subframe according to the determined allocationconditions (i.e., the limitation target subframe, the upper limit valueof the PDCCH usage in the limitation target subframe, and the on/off ofthe linked-control of the PDSCH usage in the limitation targetsubframe).

The radio resource allocation unit 285 controls the radio communicationunit 210 so that the radio terminal MUE is notified of the allocationinformation for one subframe by using a PDCCH of a subframe immediatelybefore the one subframe.

The transmission power controller 286 controls downlink transmissionpower of PDCCH and/or PDSCH for each radio terminal MUE connecting withthe macrocell base station MeNB according to the determined transmissionpower conditions (i.e., the limitation target subframe, the upper limitvalue of the PDCCH usage in the limitation target subframe, the RNTPthreshold, and the on/off of the linked-control of the PDSCH usage inthe limitation target subframe).

When the linked-control of the PDSCH is on, the transmission powercontroller 286 limits the downlink transmission power of all the RBs inthe data region in the limitation target subframe to the RNTP thresholdor below. Also, when the linked-control of the PDSCH is on, thetransmission power controller 286 limits the downlink transmission powerto the RNTP threshold or below for only part of the RBs in the dataregion in the non-limitation target subframe of the macrocell basestation MeNB.

On the other hand, when linked-control of the PDSCH is off, thetransmission power controller 286 limits the downlink transmission powerto the RNTP threshold or below for only part of the RBs in the dataregion in each of the limitation target subframe and the non-limitationtarget subframe of the macrocell base station MeNB.

(2.2) Configuration of Picocell Base Station

FIG. 21 is a block diagram showing the configuration of a picocell basestation PeNB according to the seventh embodiment.

As shown in FIG. 21, the controller 120 of the picocell base stationPeNB according to the seventh embodiment has a target subframedetermination unit 181, an upper limit value determination unit 182, aPDSCH linked-control determination unit 183, a notification generator184, a transmission power controller 185, and a radio resourceallocation unit 186.

The target subframe determination unit 181 determines a limitationtarget subframe in which a PDSCH usage of the picocell base station PeNBis to be limited to the upper limit value or below among multiplesubframes to be used in the future. Note that the term “usage” containsthe concepts of “usage rate” and “transmission power.” Specifically, theusage (in other words, energy) is determined by a product of the usagerate and the transmission power.

The target subframe determination unit 181 may use a subframe other thanthe limitation target subframe of the macrocell base station MeNB as alimitation target subframe of the picocell base station PeNB based onthe information indicating the limitation target subframe contained inthe notification message received by the X2 interface communication unit140 from the macrocell base station MeNB.

The upper limit value determination unit 182 determines an upper limitvalue of the PDCCH usage in the limitation target subframe. Also, in thepresent embodiment, the upper limit value determination unit 182determines an RNTP threshold according to the upper limit value of thedetermined PDCCH usage. Note that the upper limit value determinationunit 182 may determine the RNTP threshold of the picocell base stationPeNB based on information different from the information on the upperlimit value of the determined PDCCH usage.

The PDSCH linked-control determination unit 183 determines whether tomake the linked-control of the PDSCH usage of the picocell base stationPeNB in the limitation target subframe determined by the target subframedetermination unit 181. When the linked-control of the PDSCH usage ison, the downlink transmission power of all the RBs in the limitationtarget subframe of the picocell base station PeNB is limited to the RNTPthreshold or below. Also, in the non-limitation target subframe of thepicocell base station PeNB, the downlink transmission power for onlypart of the RBs in the data region is limited to the RNTP threshold orbelow.

The notification generator 184 generates a notification messagecontaining the information indicating the limitation target subframedetermined by the target subframe determination unit 181, theinformation indicating the upper limit value determined by the upperlimit value determination unit 182 and the RNTP threshold, and the RNTPinformation (the bit string associated with RBs).

When the linked-control of the PDSCH usage is determined to be on, theRNTP information for the limitation target subframe indicates that thedownlink transmission power is limited to the RNTP threshold or belowfor all the RBs in the data region of the limitation target subframe ofthe picocell base station PeNB. On the other hand, when thelinked-control of the PDSCH usage is determined to be off, the RNTPinformation for each of the limitation target subframe and thenon-limitation target subframe of the picocell base station PeNBindicates that the downlink transmission power is limited to the RNTPthreshold or below for only part of the RBs in the data region.

Note that the notification message may further include an ID showing asource base station (or a source cell) and an ID showing a transmissiondestination base station (or a transmission destination cell).

The notification generator 184 controls the X2 interface communicationunit 140 so that the generated notification message is transmitted tothe picocell base station PeNB. The X2 interface communication unit 140(and the notification generator 184) configures a transmitter totransmit control information in the present embodiment.

The radio resource allocation unit 185 allocates PDSCH and PDSCH withrespect to each radio terminal PUE connecting with the picocell basestation PeNB for each subframe according to the determined allocationconditions (in other words, the limitation target subframe, the upperlimit value of the PDCCH usage in the limitation target subframe, andthe on/off of the linked-control of the PDSCH usage in the limitationtarget subframe). The radio resource allocation unit 185 controls theradio communication unit 110 so that the radio terminal PUE is notifiedof the allocation information for one subframe by using a PDCCH of asubframe immediately before the one subframe.

The radio resource allocation unit 185 preferentially allocates thePDCCH and PDSCH to the radio terminal PUE in the cell edge of thepicocell base station PeNB in the limitation target subframe of themacrocell base station MeNB.

The transmission power controller 186 controls the downlink transmissionpower of PDCCH and/or PDSCH for each radio terminal PUE connecting withthe picocell base station PeNB according to the determined downlinktransmission power conditions (in other words, the limitation targetsubframe, the upper limit value of the PDCCH usage in the limitationtarget subframe, the RNTP threshold, and the on/off of thelinked-control of the PDSCH usage in the limitation target subframe).

When the linked-control of the PDSCH link is on, the transmission powercontroller 186 limits the downlink transmission power to the RNTPthreshold or below for all the RBs in the data region in the limitationtarget subframe. Also, when the linked-control of the PDSCH link is on,the transmission power controller 186 limits the downlink transmissionpower to the RNTP threshold or below for only part of the RBs in thedata region in the non-limitation target subframe of the picocell basestation PeNB.

On the other hand, when the linked-control of the PDSCH link is off, thetransmission power controller 186 limits the downlink transmission powerto the RNTP threshold or below for only part of the RBs in the dataregion in each of the limitation target subframe and the non-limitationtarget subframe of the picocell base station PeNB.

(2.3) Operational Example of Radio Communication System

FIG. 22 is a drawing for illustrating an operational example of theradio communication system according to the seventh embodiment.

As shown in FIG. 22, each of subframes SF#1 to SF#3 arranged in the timedirection is divided into a control region C and a data region D.

The macrocell base station MeNB determines the subframe SF#2 as alimitation target subframe of the macrocell base station MeNB,determines the subframes SF#1 and SF#3 as non-limitation targetsubframes, and determines to make the linked-control of the PDSCH in thelimitation target subframe. In the control target subframe SF#2 of themacrocell base station MeNB, part of the control region is blank. Also,the macrocell base station MeNB determines to limit the downlinktransmission power to the RNTP threshold or below for each RB on thehigh-frequency side of the data region D in each of the subframes SF#1and SE#3, and determines to limit the downlink transmission power to theRNTP threshold or below for all the RBs in the data region of thesubframe SF#2. The determination contents made by the macrocell basestation MeNB are notified in advance to the picocell base station PeNBthrough the notification message.

The picocell base station PeNB knows the determination contents made bythe macrocell base station MeNB through the notification message fromthe macrocell base station MeNB. The picocell base station PeNBdetermines that each RB on the high-frequency side of the data regionreceives a small interference from the macrocell base station MeNB inthe subframes SF#1 and SF#3 and preferentially allocates this RB to theradio terminal PUE. However, the subframes SF#1 and SF#3 may receive alarge interference from the macrocell base station MeNB in the controlregion.

Also, the picocell base station PeNB determines that each of the controlregion and the data region of the subframe SF#2 receives a smallinterference from the macrocell base station MeNB and preferentiallyallocates a radio terminal PUE (in particular a radio terminal PUE inthe cell edge) in the control region and the data region of the subframeSF#2. The subframe SF#2 has a low possibility of receiving theinterference from the macrocell base station MeNB in the control regionand has a preferred PDCCH state even in the radio terminal PUE in thecell edge, so that the radio terminal PUE in the cell edge canpreferably receive the control information from the picocell basestation PeNB. Also, the subframe SF#2 has a low possibility of receivingthe interference from the macrocell base station MeNB even in the dataregion, so that the radio terminal PUE in the cell edge can preferablyreceive user data from the picocell base station PeNB.

As described above, in the seventh embodiment, when the downlinktransmission power is limited to the RNTP threshold or below for all theRBs in the data region in the limitation target subframe of themacrocell base station MeNB, the macrocell base station MeNB transmitsthe transmission power information indicating that the downlinktransmission power is limited to the RNTP threshold or below for all theRBs in the data region of the limitation target subframe to the picocellbase station PeNB through the inter-base station communication.Accordingly, the picocell base station PeNB can know that theinterference to be received by the PDSCH of the picocell base stationPeNB is small in the limitation target subframe of the macrocell basestation MeNB, so that the PDSCH can be effectively allocated.

Note that although the seventh embodiment describes the configurationcapable of determining whether to make the linked-control of the PDSCH,but may have a configuration in which the linked-control of the PDSCH isalways on in a limitation target subframe, in other words, thetransmission power is limited by the RNTP threshold in the limitationtarget subframe. In this case, the PDSCH linked-control determinationunits 283, 183 are unnecessary.

Also, in another employable method, an RNTP threshold to be notified isset to be 0 dB to indicate that “the linked-control of the PDSCH is off”and an RNTP threshold to be notified is set to be other than 0 dB toindicate that “the linked-control of the PDSCH is on.”

Other Embodiments

As described above, the present invention has been described by usingthe above-described embodiments. However, it should not be understoodthat the description and the drawings, which constitute part of thisdisclosure, are to limit the present invention. Various alternativeembodiments, examples, and operational techniques will be obvious forthose who are in the art from this disclosure.

For example, each of the above-described first to seventh embodimentsmay be implemented as an independent embodiment but also as a combinedembodiment.

The above-described embodiment describes the example in which as thePDCCH usage, the number of OFDM symbols in the time axis direction whichis capable of being used as PDCCH in a downlink subframe (in otherwords, the number of OFDM symbols in the time axis direction in thecontrol region) is designated in a range of 1 to 3 (or 1 to 4), or theexample in which a usage rate being a ratio of occupying a used PDCCHresource to the entire PDCCH domain (the control region) in one subframeis designated. However, not limited to these designation methods, suchmethod can be designated that the total number of recourses capable ofbeing used as a PDCCH in a downlink subframe is designated by the numberof OFDM symbols (in other words, the total number of OFDM symbols in thecontrol region).

Although each of the above-described embodiments describes the casewhere the interference is suppressed by controlling the PDCCH usage, butsuch method may be used that the interference is suppressed bycontrolling not only the PDCCH usage but also PDCCH transmission power.The PDCCH transmission power is transmission power in the entire PDCCHdomain (the control region) in one subframe, in which, for example,transmission power for each resource element which is a resource minimumunit is averaged by the entire PDCCH domain (the control region). ThePDCCH interference between the base stations can be suppressed even when“PDCCH usage” in each of the above-described embodiments is read as“PDCCH transmission power.”

The first, third, fourth, fifth, sixth, and seventh embodiments describethe technique of reducing the inter-base station PDCCH interferencebetween the macrocell base station and the picocell base station, andthe second embodiment describes the technique of reducing the inter-basestation PDCCH interference between the macrocell base stations. However,the present invention is not limited to these combinations of basestations, and can be applied to the reduction of the inter-base stationPDCCH interference between any base stations.

Also, it is anticipated in the LTE Advanced that a relay node being aradio base station having a radio backhaul configuration is employed andan X2 interface is also employed for the relay node. Thus, the relaynode may be a radio base station according to the present invention.

Furthermore, the above-described embodiments describe the LTE system,but the present invention may be applied to another radio communicationsystem such as a radio communication system based on WiMAX (IEEE802.16).

As described above, it should be understood that the present inventionincludes various embodiments not described herein. Accordingly, thepresent invention is only limited by the scope of claims and mattersspecifying the invention, which are appropriate from this disclosure.

Note that the contents of Japanese Patent Application Publications No.2010-87686 (filed on Apr. 6, 2010), No. 2010-138802 (filed on Jun. 17,2010), No. 2010-181165 (filed on Aug. 12, 2010), and No. 2010-225266(filed on Oct. 4, 2010) are hereby incorporated by reference in theirentirety.

INDUSTRIAL APPLICABILITY

As described above, a radio communication system, a radio base station,and a communication control method according to the invention can reducean inter-base station interference between downlink control channels,and therefore are useful in radio communications such as a mobilecommunication.

The invention claimed is:
 1. A first radio base station comprising acontroller including a processor configured to: perform a downlinkcommunication using a radio frame configuration comprising a pluralityof subframes arranged in a time direction, wherein each of the pluralityof subframes comprises a plurality of resource blocks arranged in afrequency direction; receive, from a second radio base station which isreceiving an interference caused by the first radio base station, via anX2 interface, a request for transmission of a message including a bitstring comprising a plurality of bits corresponding to the plurality ofsubframes, wherein each bit position of the plurality of bits representsa different subframe number; and in response to receiving the requestfrom the second radio base station, transmit, to the second radio basestation via the X2 interface, the message including the bit stringcomprising the plurality of bits, wherein the plurality of bitscomprise: “1” bit that indicates a restricted subframe in which thefirst radio base station restricts a downlink transmission power below apredetermined value, among the plurality of subframes; and “0” bit thatindicates a non-restricted subframe in which the first radio basestation does not restrict a downlink transmission power below thepredetermined value, among the plurality of subframes.
 2. A second radiobase station comprising a controller including a processor configuredto: transmit, to a first radio base station which is causing aninterference to the second radio base station, via an X2 interface, arequest for transmission of a message including a bit string; and inresponse to transmitting the request to the first radio base station,receive the message including the bit string from the first radio basestation via the X2 interface, wherein the first radio base stationperforms a downlink communication using a radio frame configurationcomprising a plurality of subframes arranged in a time direction,wherein each of the plurality of subframes comprises a plurality ofresource blocks arranged in a frequency direction, the bit stringcomprises a plurality of bits corresponding to the plurality ofsubframes, each bit position of the plurality of bits represents adifferent subframe number, and the plurality of bits comprise: “1” bitthat indicates a restricted subframe in which the first radio basestation restricts a downlink transmission power below a predeterminedvalue, among the plurality of subframes; and “0” bit that indicates anon-restricted subframe in which the first radio base station does notrestrict a downlink transmission power below the predetermined value,among the plurality of subframes.
 3. A communication control method usedin a radio communication system comprising: performing, at a first radiobase station, a downlink communication using a radio frame configurationcomprising a plurality of subframes arranged in a time direction,wherein each of the plurality of subframes comprises a plurality ofresource blocks arranged in a frequency direction; transmitting, from asecond radio base station which is receiving an interference caused bythe first radio base station, to the first radio base station via an X2interface, a request for transmission of a message including a bitstring corresponding to the plurality of subframes; receiving, at thefirst radio base station, the request from the second radio basestation; in response to receiving the request from the second radio basestation, transmitting, from the first radio base station to the secondradio base station via the X2 interface, the message including the bitstring comprising the plurality of bits, wherein each bit position ofthe plurality of bits represents a different subframe number, andwherein the plurality of bits comprise “1” bit that indicates arestricted subframe in which the first radio base station restricts adownlink transmission power below a predetermined value, among theplurality of subframes, and “0” bit that indicates a non-restrictedsubframe in which the first radio base station does not restrict adownlink transmission power below the predetermined value, among theplurality of subframes; and receiving, at the second radio base station,the bit string from the first radio base station via the X2 interface.